Scientists are divided on the precise relation between Active Sleep and REM sleep. Some suggest that they are similar, while others say it is an entirely different state, which represents aspects of the developing CNS that are not present in a mature brain, and that certain aspects of REM are not present in Active Sleep due to the immaturity of the CNS in the neonate.

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While it depends on age, neonatal sleep is sometimes scored as Active Sleep, Quiet Sleep, and Wake. This is less specific than the classification of adult's sleep, and is often based on behavioral criteria due to the technical difficulties arising from recording EEG from the neonate.

The periodicity of the sleep cycle is strongly correlated with body size both between and within species. This may be related to the demands of thermoregulation. During quiet sleep the thermoregulatory mechanisms work normally but there is no response to thermal stress during active sleep. This could have serious implications for small animals (see gigantothermy) whose body temperature is more easily influenced by ambient temperature because of their lesser thermal capacity. So, as a rule, smaller animals have shorter periods of active sleep, ending before becoming thermally threatening.[1]